1. Field of the Invention
This invention relates to angular position sensing transducers and more particularly to capacitive position sensing transducers.
2. Description of the Prior Art
Angular shaft position sensing devices have been extensively used for many years. Traditionally, such position sensing transducers have been either magnetic sensing transducers or optical sensing transducers. However, in the past decade, capacitive position sensing transducers have been increasing in interest to the art. Capacitive transducers have been less sensitive to stray noise than either magnetic or optical transducers. Unlike the optical transducer, the capacitive transducers are not sensitive to ambient light and unlike the magnetic transducer, the capacitive transducer is not sensitive to stray magnetic fields. In addition, the capacitive transducer is less sensitive to the effects of accumulated dust than the other transducers. Also, optical transducers have a greater operating life than other transducers. Capacitive transducers are lower in cost than optical transducers.
Some representative prior art description of capacitive transducer technology may be found in the following:
"Dual Plane Capacitive Coupling Encoder", authored by R. J. Flaherty, M. L. Sendelweck, and J. W. Woods, IBM Technical Disclosure Bulletin, Vol. 15, No. 4, September 1972.
"Electrodynamic Velocity and Position Sensor and Emitter Wheel", authored by H. E. Naylor, III, and R. A. Williams, IBM Technical Disclosure Bulletin, Vol. 16, No. 10, March 1974.
U.S. Pat. No. 3,702,467, "Shaft Position Sensing Device", George Melnyk, Issued Nov. 7, 1972.
U.S. Pat. No. 3,938,113, "Differential Capacitive Position Encoder", D. R. Dobson et al, Issued Feb. 10, 1976.
U.S. Pat. No. 4,040,141, "Twin-Capacitive Shaft Angle Encoder with Analog Output Signal", J. C. Fletcher et al, Issued Aug. 2, 1977.
In substantially all systems involving rotational shaft positioning, e.g., stepper motors, a homing procedure must be provided. In both closed loop stepper motor systems where the position of the stepper motor is dependent upon transducer sensed feedback and in open loop stepper motor systems where shaft positioning is not dependent on such feedback, means must be provided for sensing the home position of the stepper motor. In rotational shaft positioning systems such as stepper motors, the capability of quickly and easily sensing the home position is very important in initializing system operation, periodically checking on shaft positions and for various forms of equipment diagnostics.
In most magnetic and optical shaft position sensing systems, it has become conventional to provide a magnetic or optical transducer device for sensing the home position which is separate from the transducer device which senses all the other positions. Since all position sensing with magnetic or optical transducers is dependent upon a count of sensed positions from the initial or home position, a separate home positioning sensing device is generally used to determine the home position from which the other position sensing device may commence its positional count. It should be noted that apparatus which uses a magnetic or optical device to sense the various positions of the shaft need not use a corresponding optical or magnetic device for sensing the home position; home position could be sensed with a simple electrical contact or a mechanical detent.
While as set forth hereinabove, capacitive transducers have been of increasing importance in sensing rotational shaft positions, the use of such transducers for sensing home positions has presented a problem. Unlike optical or magnetic transducers wherein a single indicator at the home position will provide a sufficiently discernible signal so that the home position can be determined, in capacitive sensing, a single indicator at the home position will customarily produce a signal which is so weak that it may not be discernible from general noise.